Sceptic acting so ridiculously (8)
I believe the answer is:
agnostic
'sceptic' is the definition.
(I've seen this before)
'acting so ridiculously' is the wordplay.
'ridiculously' indicates anagramming the letters.
'acting'+'so'='actingso'
'actingso' is an anagram of 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "He doesn't believe" , "One who is undecided" , "A non-believer in God, coasting along" , "One believes that nothing can be known about the existence of God" , "Not sure" .)